Parse error when switched to PHP 7.3

Hello 🙂 I have a problem on my website. Everything was OK while using PHP 5.6 version on my webhosting. I changed to PHP 7.3 and there a lot of PHP warning errors like Warning: Decalaration of AtomWalkerTerms::start_lvl.
I want new PHP because even wordpress show warning on the dashboard.

I proceeded as follows

Declaration of AtomWalkerTerms::start_lvl(&$output, $depth, $args)
should be compatible with
Walker_Category::start_lvl(&$output, $depth = 0, $args = Array) in /mydomain/wp-content/themes/mystique/atom-core.php on line 261

so I searched for line 261 where is AtomWalkerTerms and found this:
function start_lvl(&$output, $depth, $args){
and changed to this:
function start_lvl(&$output, $depth = 0, $args = Array)

I fixed all of them but then will show Parse error
Parse error: syntax error, unexpected ')', expecting '(' in

even If I remove the whole line, then another lines keeps appearing. And then again lines that was already fixed shows problems.

I also tried turn on WordPress debugging and it shows warnings for two plugins plugins Crafty social buttons and ChimpMate: Deprecated: Function create_function()

Can somebody help me? Thanks in advance

Screenshots: https://imgur.com/a/rtoNC86

I found on StackEchange user JHoffmann wrote:
This issue existed before PHP7 but in earlier versions, the warnings emitted had an error level of E_STRICT. Since PHP 7 they are normal warnings, hence they are shown with default error level setting

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